Les cahiers de l'ASN #03 - 10 years after Fukushima

28 march 1979 THREE MILE ISLAND REACTOR STATUS AT THE BEGINNING OF THE ACCIDENT Automatic shutdown of pressurised water reactor (PWR) N° 2 owing to a failure of the normal water supply to the steam generators. MAIN CAUSES OF THE ACCIDENT Combination of failures (steam generators normal feedwater system and system which was supposed to back it up, as a result of incorrect reconfiguration following a test). Information on the status of the safety equipment not available (position of the pressuriser letdown valve). Loss of reactor core cooling (water injection stopped) owing to an incorrect diagnostic, which led to core heating, uncovering and then partial melting. LESSONS LEARNED This accident showed that combinations of failures could lead to core melt and how important it is for control crews to have information about the status of the facility . Insufficient account was taken of human and organisational factors. Humans are however an essential link in the safety chain . Following this accident, changes were made to the installations, in particular additional measures to manage combinations of failures, means to mitigate releases of radioactive products outside the containment (see diagram) and the creation of emergency plans and emergency exercises to test them. When they occurred, the major nuclear accidents of Three Mile Island in the United States and Chernobyl in Ukraine were analysed by ASN and its technical support body, IRSN*, and were debated within international organisations, such as the IAEA*. The lessons learned led to significant progress being made in the safety of nuclear facilities. LES GRANDS ENSEIGNEMENTS DES AUTRES CATASTROPHES SONS LEAR ED FRO O HER NUCLEAR ACCIDENTS A look back at two significant accidents * See glossary page 24 Radioactive gas Valve Stack Sand Expanded clay Concrete floor Reactor building Drain Inside the stainless steel shell, the gases pass through different layers, including 80 cm of sand. A SIGNIFICANT STEP FORWARD IN SAFETY Containment air filtration In the event of an accident, if a pressure increase were to threaten damaging the containment, the depressurisation system would, as a last resort, enable the gases in the containment to be released after filtration. This filter is capable of retaining some of the radioactivity and thus mitigate the environmental consequences of the accident. 22 • Les cahiers de l’ASN • March 2021